Homing to sides of workpiece with real tool How? Lathe

andypugh wrote:

berntd wrote:

My machine does not have switches and most machines apparently do not and I also can't see a reason for them.

What makes you think that most machines don't have home switches? I consider them all but essential, especially on a lathe.

4) I move to the right, clear of the workpiece.with my tool in the toolpost.

Which tool? I find it useful to have a "reference tool" which always has zeros in the tool table.

6) When the tool reaches the feeler gauge, I home the Z axis.

You should home to a fixed point on the axis, not to the work. Homing to the work will mean that the G54 offsets will move around.

There are three sets of offsets in play here. When you turn on the machine the step-generators are at zero. Homing sets the offsets added to that position to convert it to absolute machine coordinates. If you switch to viewing absolute coordinates you will see these go to zero when you home. (well, actually, the will go to a position set by the HOME_OFFSET in the INI file)
Then on top of this there is the G54 (G55, G56....) coordinate offset. This converts absolute machine positions to suit you G-code origin.
Then there is the tool table offset, added on when you load a tool and apply the offsets with G43

7) I move the tool out and to the left so I can move towards the OD, again with a feeler gauge.
8) When reached, I zero the X axis.

I don't know if this matters, but that is unusual. Normally you would set the X to the actual radius (or diameter) of the work + feeler gauge.

This should, in my opinion, put the machine 0,0 outside my workpiece in a safe place. (Currently this does not produce a 0,0 on the coordinates. I cannot explain that. I did before)

Are you viewing the relative or the absolute coordinates?

9) I enter T2 M6 to load tool 2
10) I I click tool touch off for X and enter 0.1 into the box.
11) I click same for Z and also enter 0.1 into the box.

You may also need a G43. With Touchy you need to issue a G43 after a touch-off, it is possible that Axis does it for you, I can't remember.
The various variants of "touch off" do one of the 4 versions of G10. (G10 L1, G10 L10, G10 L2, G10 L20) Which one you get depends on whether you touch off the coordinate system or the tool table, and whether you are set to touch-off to work or to fixture.
linuxcnc.org/docs/2.7/html/gcode/g-code.html#gcode:g10-l1

Now I should be set to go. When I go to 0 on either axis, the tool will touch the workpiece either on the face or the OD.

It all seems to work ok. Until I now restart the whole process.from step 1..

Re-homing with the axes in a different physical position will move _everything_. This means that different tool and G54 offsets will be calculated in the later stages.

Andy I will print this out and go through it step by step at the machine as soon as possible.

Switches:
I have followed the advice of Joe Martin, founder of Sherline Inc, in that whilst limit/home switches can stop a machine running into its end stops, small machines like my Sherline do not have to have them. In fact, they do not even supply any for the lathe as far as I know. Switches do not prevent workpiece damage or crashing since the path taken to the home position can easily be through the part. Whilst I reckon they are useful, they will not help me solve this here problem.

You can add your own. Doesn't need to be anything fancy to get you going. Optical would be preferable if accuracy is a must but simple micro switches will do. Do you have 2 input pins available? There is also no reason why a tool would home through your part. You can configure your ini file so it homes properly. The other nice thing is when the soft limits are set, you can jog towards the end of travel and it knows when to stop. The machine when configured properly will know it's work envelope. Not to mention, the next day when you want to make more of the same parts, you just home the machine, load the stock and program, and hit cycle start. You don't need to go through the setup procedure again.

andypugh wrote:

[quote="OT-CNC" post=118888You can add your own. Doesn't need to be anything fancy to get you going.

Actually, it's a Sherline. A perfectly adequate way to home it would be to manually wind the axes to the hard stops before pressing the "home-all" button.

Then you can touch off to the wokpiece with a master tool, and then you can touch-off the other tools.[/quote]

Indeed, + the hard stops change, depending where the tailstock is sitting.
But we are digressing from the actual problem at hand...

the hard stops change, depending where the tailstock is sitting.

You can place the home switch on the bed with no tailstock issues, it does not matter where that point is on the bed.
Softlimits can be ± from that point. It is far better to have a fixed home position that not having it at all, as I said before you are in deep waters without home position! All depending of what you are doing of course but in general always see to have home switches, makes your life so much easier to have repeatability.

On my lathe now I have home position for Z about 3/4 bed length away from the chuck and X max +. This is 100% repeatable!

Thanks. Good idea! But what about my pressing problem with the touch off not producing the right results?

Can you show us with text and pictures how you home and how you touch off to part.
I suspect you are doing something wrong.

Finally some more time to work on this problem today...
The problem still exists bu the cause may actually be something else.

I may need to start a new thread but I will try here first:
As follows:
1) I mounted a dial gauge on the Z-axis.
2) I move the axis (jog slowly) to the right and stop.
3) I home the axis in this position.
3) I set the dial gauge to 0.
4) I move the axis left with G01 Z-1.000
5) I find the axis has actually moved to -1.1200 (all mm)
6) I move the axis back to 0 with G01 Z0.
7) I find the axis has actually moved to -0.1200 (all mm)

I have a 0.12mm error and it is permanent!
Repeating steps 4-6 many times will not shift anything nor cause creeping. It is permanently off by 0.12mm.


ADDITION:
I just discovered that when I press "Home Axis", The Z axis coordinate does not become 0 as expected but 0.120.
I think this is related to the problem

There is no tool loaded.
Bottom right of window states: Position: Relative Actual

ADDITION2:
I now think this is still a problem with the G54 coordinates being offset by an amount that I cannot explain. I do not know how it got there nor how to get rid of it.

If I can solve this problem, The homing and touch off issues may also be resolved.

Any ideas anyone?

Regards
Bernt

Thanks. I think understand this

What I do not understand is why the touch off offset seems to become incorrect if one repeats the touch off operation

I touched off at a point for Z and entered -7.33. It was verified with the dial gauge.
I then went to 0 with g01 z0.
dial gauge read -0,120.
That would only make sense if the touch off accidently operated on the machine coordinates. But this is not the case since I operated in g54.

Repeating the touch off results in further strange offsets when going to z0.

I have ruined 2 worpieces with this so far and I still have no explanation.

I wanted to video it but my new gopro is not compaible with any android device I own.